Prenatal Summary

Pregnancy begins at conception with the union of a man’s sperm and a woman’s egg to form a single-cell embryo.1This brand new embryo contains the original copy of a new individual’s complete genetic code. Gender,2 eye color, and other traits are determined at conception, also known as fertilization.

Most significant developmental milestones occur long before birth during the first eight weeks following conception when most body parts and all body systems appear and begin to function.3

The main divisions of the body, such as the head, chest, abdomen and pelvis, and arms and legs are established by about four weeks after conception.4 Eight weeks after conception, except for the small size, the developing human’s overall appearance and many internal structures closely resemble the newborn.5

Pregnancy is not just a time for growing all the parts of the body. It is also a time of preparation for survival after birth.6 Many common daily activities seen in children and adults begin in the womb—starting more than 30 weeks before birth. These activities include hiccups, touching the face, breathing motions, urination, right- or left-handedness, thumb sucking, swallowing, yawning, jaw movement, reflexes, REM sleep, hearing, taste, sensation, and so on.

Full-term pregnancy typically lasts 38 weeks from conception or 40 weeks from the first day of a woman’s last normal menstrual period.7

Unless otherwise noted, all prenatal ages on this web page are referenced from the start of the last normal menstrual period. This age is two weeks greater than the age from conception, also referred to as fertilization. (Please note that on the remainder of ehd.org, prenatal ages are referenced from the time of conception.)

The First Two Weeks

Shortly after a woman’s period begins, her body begins preparing for the possibility of pregnancy.

Approximately 2 weeks into her cycle, a woman releases an egg from one of her ovaries into her adjacent fallopian tube. Conception is now possible for the next 24 hours or so8 and signifies the beginning of pregnancy.9

The single-cell embryo has a diameter of approximately 4 thousandths of an inch.10

2 to 4 Weeks

The cells of the embryo repeatedly divide as the embryo moves through the Fallopian tube into the woman’s uterus or womb. Implantation, the process whereby the embryo embeds itself into the wall of the womb, begins by the end of the third week and is completed during the fourth week of pregnancy.11

4 to 6 Weeks

By 5 weeks, development of the brain, spinal cord,12 and heart13 is well underway.

The heart begins beating at 5 weeks and one day14 and is visible by ultrasound almost immediately.15

By 6 weeks, the heart is pumping the embryo’s own blood to his or her brain and body.16 All four chambers of the heart are present17 and more than 1 million heartbeats have occurred.18 The head, as well as the chest and abdominal cavities have formed19 and the beginnings of the arms and legs are easily seen.20

The 6-week embryo measures less than ¼ of an inch long from head to rump.

8 to 10 Weeks

Also by 8½ weeks, the bones of the jaw and collar bone begin to harden.25

By 9 weeks the hands move, the neck turns,26 and hiccups begin.27 Girls now have ovaries28 and boys have testes.29 The embryo’s heart rate peaks at about 170 beats per minute30 and will gradually slow down until birth.

Electrical recordings of the heart at 9½ weeks are very similar to the EKG tracing of a newborn.31 The heart is nearly fully formed.32

By 10 weeks kidneys begin to produce and release urine,33 and intermittent breathing motions begin.34 All fingers and toes are free and fully formed,35 and several hundred muscles are present.36 The hands and feet move frequently and most embryos show the first signs of right- or left-handedness.37

Experts estimate the 10-week embryo possesses approximately 90% of the 4,500 body parts found in adults.38 This means that approximately 4,000 permanent body parts are present just eight weeks after conception.

Incredibly, this highly complex 10-week embryo weighs about 1/10th of an ounce and measures slightly less than 1¼ inches from head to rump.

10 to 12 Weeks

By 11 weeks the head moves forward and back, the jaw actively opens and closes, and the fetus periodically sighs41 and stretches.42 The face, palms of the hands, and soles of the feet are sensitive to light touch.43 Thumb sucking44 and swallowing amniotic fluid begin.45 Girls’ ovaries now contain reproductive cells46 which will give rise to eggs later in life. Also in girls, the uterus is now present.47

16 to 18 Weeks

Around 17 weeks blood cell formation moves to its permanent location inside the bone marrow62 and the fetus begins storing energy in the form of body fat.63

By 18 weeks formation of the breathing passages, called the bronchial tree, is complete.64 The fetus releases stress hormones in response to being poked with a needle.65

The 18-week fetus weighs around 6 ounces and measures about 8 inches from head to heel.

18 to 20 Weeks

By 19 weeks, more than 20 million heartbeats have occurred.

By 20 weeks the larynx or voice box begins moving in a way similar to the movement seen during crying after birth.66 The skin has developed sweat glands67 and is covered by a greasy white substance called “vernix,”68 which provides protection from the amniotic fluid.69

The 20-week fetus weighs about 9 ounces and measures about 10 inches from head to heel.

20 to 22 Weeks

By 22 weeks the sense of hearing begins to function and the fetus starts responding to various sounds.71 The cochlea, the organ of hearing, reaches adult size.72 All skin layers and structures are complete.73

With specialized medical care some fetuses can survive outside the womb by 22 weeks with survival rates reported as high as 40%74 in some medical centers.

The 22-week fetus weighs slightly less than 1 pound and measures about 11 inches from head to heel.

22 to 24 Weeks

Between 20 and 23 weeks rapid eye movements begin. These eye movements are similar to those seen when children and adults have dreams.75

By 24 weeks more than 30 million heartbeats have occurred.

The 24-week fetus weighs about 1¼ pounds and measures about 12 inches from head to heel.

38 to 40 Weeks

Labor is initiated by the fetus,89 ideally around 40 weeks, leading to childbirth.

At full-term birth, newborn babies typically weigh between 6 and 9 pounds and measure between 18 and 21 inches from head to heel.90

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The Endowment for Human Development, Inc. (EHD) hereby grants state and local governments a non-exclusive limited license to incorporate any or all of the prenatal development facts presented on this “Prenatal Summary” web page (English and Spanish) into government web pages and printed materials at no charge. Government entities using EHD material on the Internet are required to acknowledge EHD in the credits and provide at least one live link to this Prenatal Summary web page (http://www.ehd.org/prenatal-summary.php). Similarly, governments using EHD development facts in printed materials should list EHD as a source in the credits along with the web address of this page.

Vindla and James, 1995. 598; Carlson, 2009. 490. "Fetal breathing movements are essential for postnatal survival. One function of fetal breathing is to condition the respiratory muscles so they can perform postnatal contractions."

Implantation begins with attachment of the embryo about 6 days after fertilization. [Attachment of the embryo to the inner wall of the uterus is a transient event and is the hallmark of Carnegie Stage 4.] O'Rahilly and Müller, 2001. 40; Adams, 1960. 13-14.

Poissonnet et al., 1983. 7; Poissonnet et al., 1984. 3. In a study of 488 fetuses, Poissonnet's group found that adipose tissue (fat) appears in the face from 14 weeks postfertilization. By 15 weeks, fat appears in the abdominal wall, back, kidneys, and shoulders. By 16 weeks, fat is also present throughout the upper and lower limbs.

DiFiore and Wilson, 1994. 221-222. [There is some disagreement among experts regarding when the bronchial tree is complete. Some say completion occurs as early as 16 weeks postfertilization while others say it occurs after birth.]; Pringle, 1988. 178.

Birnholz and Benacerraf, 1983. 517; Campbell, 2002. 3. Professor Stuart Campbell correctly points out that the eyes of the fetus are closed most of the time and a true blink requires the eyes to be open. Perhaps the "blink-startle" response would be more accurately termed "squint-startle."; Drife, 1985. 778.